424 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



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the air-bath could be raised to any desired value less than 60° C. In 

 order to keep the air in the air-bath homogeneous as to temperature, 

 a fan-motor which ran at fairly constant speed was placed at each of 

 its ends. These motors stood just outside of the air-bath, and their 

 shafts, each carrying a fan, projected into the interior. 



For the regulation of the temperature of the air in the air-bath a 

 modified form of the usual device, depending upon the expansion of 



alcohol, was used. A 

 brass tube, with thin 

 walls and an internal 

 diameter of about 0.7 

 cm., was wound in the 

 form of a spiral 50 cm. 

 long and 6 cm. in diam- 

 eter. One end being 

 closed, the other was 

 connected by means of 

 a short piece of stout 

 rubber tubing to the 

 larger end of the glass 

 tube, GADB (Figure 2). 

 It was found better to 

 make the spiral of brass 

 than of glass. The brass, 

 having a lower specific 

 heat and a higher ther- 

 mal conductivity than 

 the glass, allowed the 

 liquid which it con- 

 tained to assume more 

 rapidly the temperature 

 of the air which sur- 

 rounded it. The large 

 surface which the spiral presented, in comparison with the volume of 

 liquid which it contained, about 150 (cm.)^ increased the rapidity 

 with which the liquid assumed the temperature of the surrounding air. 

 The spiral stood in a vertical position in the air-bath, and the glass 

 tube BD (Figure 2) was also vertical. The thermo-regulator was 

 filled with well -boiled alcohol. From the line E F around to the plati- 

 num point at B, it contained mercury. By taking out, or by adding 

 mercury or alcohol, one fixed the temperature of the air-bath at any 

 desired value. The platinum wires A and B (Figure 2) were con- 



Heating Coils 



Dynamo 



Figure 2. 



